This invention relates generally to a measuring apparatus, and more particularly to a weighing scale.
Various scales are commercially available for providing a digital or other indication of the weight of a body placed on a weighing pan of the scale. Many of the recently developed scales make use of electronic circuitry including integrated circuits and microcomputer controls for perfecting various scale operations such as weighing, counting, altering resolution, converting units, etc. In this regard, these devices comprise electronic circuitry usually including a movable transducer and mechanical interfacing means between the electronic circuitry and the weighing pan. To that end, notwithstanding the high levels of sensitivity and accuracy associated with the electronic circuitry, the overall accuracy of the scale for small weights nonetheless depends largely upon the ability of the mechanical means to avoid introducing vibrational and other errors into the weighing process.
With regard to the prior art, scales have tended to be one of several types. One type of scale, often referred to as a balance, is constructed so that the object being weighed on the scale's weighing pan applies a downward force to the free end of a lever arm. The lever arm is arranged to pivot about a fulcrum, so that the weight of the object on the weighing pan can readily be determined in terms of how much weight or force must be applied to the opposing end of the lever arm to exactly balance or offset the load on the weighing pan. Although these types of scales tend to be fairly accurate and are generally suitable for their intended purposes, they also tend to be somewhat slow in operation (e.g., by requiring the person operating the scale to move calibrated weights along the lever arm to achieve an exact balance) and are not easily adapted for use in combination with electronic measuring or indicator means.
Other prior art scales, of either mechanical or electronic construction, have typically included spring means and a movable arm arranged to pivot about a fulcrum, wherein the object being weighed applies a generally downward force against the free end of the arm, with the spring means serving to upwardly bias or resist any downward movement of the free end. Mechanical or electrical sensing means responsive to the lever arm's displacement are typically used to measure and indicate the weight of the object on the weighing pan. Although these scales are also generally suitable for their intended purposes, one major drawback associated with such scales is that their weighing accuracy, especially for small weights, often is adversely affected by vibration in the scale's environment, i.e., vibration transmitted to the scale through the supporting structure upon which the scale is resting.
Approaches to eliminating the adverse effect of vibration in the up/down direction are disclosed in applicant's U.S. patent application Ser. No. 584,416, entitled "Electronic Scale With Counterbalance". In the first embodiment disclosed in applicant's '416 application a generally horizontal, movable lever arm carrying a movable, generally horizontally disposed capacitor plate at the end thereof is counterbalanced so that the center of gravity of the lever arm and the elements supported thereon is along the pivot axis of the fulcrum of said lever arm.
The counterbalanced arrangement employed in the first embodiment disclosed in the '416 application generally is effective in minimizing weighing inaccuracies created by vibrations imparted to the movable parts of the scale in an up/down direction. However, that counterbalanced arrangement is not designed to eliminate or minimize weighing inaccuracies resulting from lateral or rotary vibrations imparted to the scale from the surrounding environment.
Vibrations that impart a rotational force to the lever arm of the first embodiment of the scale disclosed in the '416 application initially will cause the lever arm to be rotationally displaced about its fulcrum, followed by gradually attenuating rotational oscillations of said arm. Although these oscillations may be small in amplitude, the amount of error introduced into the weighing process as a result thereof often is sufficiently great to produce substantial weighing inaccuracies, particularly where lightweight objects are being weighed.
Moreover, although the adverse effects of up/down vibrations are minimized in the first embodiment of the scale disclosed in the '416 application, movable elements of the scale, including the movable horizontally disposed capacitor plate, still will move vertically to some extent under the influence of such vibrations. Such vertical movement of the capacitor plate, however small, introduces undesired errors into the weighing operation, which can be particularly significant when the precision measurement of a lightweight object is required.
A further deficiency of the first embodiment of the scale disclosed in the '416 application is that any change in the length of the lever arm or in the direction of the force applied to the pull cable will result in a corresponding change in the applied moment, any hence the perceived weight, thereby resulting in errors in the weighing operation. The length of the lever arm is equal to the distance between the axis of rotation and the center of action of the applied force (applied by the pull cable) which is applied perpendicularly to the lever arm. Ideally the pull cable will be oriented perpendicularly to the lever arm, and the center of action of the applied force will be the center line of the cable. However, in practice, the center of action of the pull force may move from the center line, either closer or further from the axis of rotation, in an unpredictable manner with time and usage of the scale. This may occur as tension shifts from one fiber to another in the pull cable, or as a result of slight slippage which may occur in the clamps holding the cable. Additionally, if the parallelogram structure of the suspension system moves slightly, due for example to slippage in its attachment points, the clamp at the bottom of the pull cable and the direction of pull may no longer be perpendicular to the lever arm. Although the change in the applied moment may be ever so slight, the amount of error introduced into the weighing process as a result of such a change often is sufficiently great to cause concern, particularly when lightweight objects are being weighed. For example, a change of only 25 mils in a 2.5" moment arm has an effect of 0.1% on the measured weight.
Flexure base scales of the type disclosed in the second embodiment of applicants '416 application employ a symmetrical arrangement of moving beams that will minimize the adverse effect of rotational and horizontal vibrations imparted to the moving elements of said scale. In the specific embodiment disclosed in the '416 application the movable beams also are counterbalanced for the purpose of minimizing the adverse effect of vibrations in the up/down direction. Although the counterbalanced flexure base scale disclosed in the '416 application has not been used, sold or otherwise disclosed in a manner to make it prior art against the invention disclosed and claimed herein, it is discussed herein because it provides a background against which the present invention can be better understood.
Although the flexure base scale disclosed in the '416 application is generally acceptable for its intended purpose, it does have certain deficiencies that make it unsuitable for the high precision measurement of lightweight objects.
First, flexure base scales are large, with the internal moving elements thereof being of a fairly high mass. Regardless of the measures taken in designing such a scale, some movement of these internal elements will occur if the scale is subject to vibrations from the surrounding environment. The fact that these elements are of a high mass construction, exacerbates the problem of achieving accuracy in the weighing operation. In other words, the adverse effects of vibrations on the accuracy of a scale can be reduced by reducing the mass of the elements that are subject to movement under the influence of such vibrations. Unfortunately, flexure base scales are inherently large scales employing high mass moving elements.
Second, electronic position sensing means of flexure base scales generally are mounted on horizontally disposed members, and are required to move vertically to provide a signal indicative of the weight of the material. Even when the counterbalanced arrangement disclosed in applicant's '416 application is employed, the horizontal mounting of the position sensing means generally will cause some undesired vertical excursions of the position sensing means to occur when vibrations are imparted to the scale in the up/down direction, and this introduces undesired errors into the weighing operation.
There also are other types of prior art scales which do not utilize a pivot arm, but which use a spring or other means to bias a platform upwardly against the downward force of a load being weighed, also are subject to weighing inaccuracies attributable to vibrations of the type described above.